Zeolites are widely used as catalysts for various types of hydrocarbon-conversion processes such as isomerization, reforming, hydrogenation, alkylation, transalkylation, cracking and hydrocracking. In addition, zeolitic materials are used as adsorbents in various petroleum and chemical separation processes. Selectivity in catalysis or separation is conferred by the interstitial spaces or channels formed by the network of crystalline aluminosilicates. Zeolites also may comprise materials in which the silica and alumina portions have been replaced in whole or in part with other oxides; germanium oxide, tin oxide, and mixtures thereof can replace the silica portion and gallium oxide, indium oxide, boron oxide, iron oxide, and mixtures thereof can replace the alumina portion.
The practical applications of pure zeolites are severely limited because of mechanical-strength limitations. Mechanical strength may be conferred by forming the zeolite in the presence of a non-zeolitic binder and drying and calcining the resulting extrudate pill, sphere, or extrudate. Examples of such binders include materials such as alumina, silica, titanium, and various types of clays. However, the effectiveness of a bound zeolite in terms of activity, selectivity, or activity maintenance, can be reduced because the binder dilutes the adsorptive properties of the zeolite. In addition, since the bound zeolite is prepared by extruding the zeolite with the binder and subsequently drying and calcining the extrudate, the amorphous binder can penetrate the pores of the zeolite, otherwise block access to the pores of the zeolite, or slow the rate of mass transfer to the pores of the zeolite which can reduce the effectiveness of the zeolite. Still further, when a bound zeolite is used in catalytic processes, the binder may affect the chemical reactions that are taking place within the zeolite and also may catalyze undesirable reactions which can result in the formation of undesirable products.
A key application of zeolitic catalysts is in the conversion of C8 aromatics to obtain individual xylene isomers. The xylenes, para-xylene, meta-xylene and ortho-xylene, are important intermediates which find wide and varied application in chemical syntheses. Para-xylene upon oxidation yields terephthalic acid which is used in the manufacture of synthetic textile fibers and resins. Meta-xylene is used in the manufacture of plasticizers, azo dyes, wood preservers, etc. Ortho-xylene is feedstock for phthalic anhydride production.
The catalytic reforming of petroleum naphtha is an important source of C8 aromatics. Xylene isomers in C8 aromatics from catalytic reforming or other sources generally do not match demand proportions as chemical intermediates, and further comprise ethylbenzene which is difficult to separate or to convert. Para-xylene in particular is a major chemical intermediate with rapidly growing demand, but amounts to only 15-20% of a typical C8-aromatics stream. Adjustment of isomer ratio to demand can be effected by combining xylene-isomer recovery, such as adsorption for para-xylene recovery, with isomerization to yield an additional quantity of the desired isomer. Isomerization converts a non-equilibrium mixture of the xylene isomers which is lean in the desired xylene isomer to a mixture which approaches equilibrium concentrations. The approach to equilibrium that is used is an optimized compromise between high C8 cyclic loss at high conversion (i.e. very close approach to equilibrium) and high utility costs due to the large recycle rate of unconverted C8 aromatics.
Processes for conversion of C8 aromatics ordinarily are classified by the manner of converting ethylbenzene associated with the xylene isomers. Ethylbenzene is not easily isomerized to xylenes, but it normally is converted in the process unit because separation from the xylenes by superfractionation or adsorption is very expensive. One approach is to react the ethylbenzene to form a xylene mixture via conversion to and reconversion from naphthenes in the presence of a solid acid catalyst with a hydrogenation-dehydrogenation function. An alternative widely used approach is to dealkylate ethylbenzene to form principally benzene while isomerizing xylenes to a near-equilibrium mixture. The former approach enhances xylene yield by forming xylenes from ethylbenzene; the latter approach commonly results in higher ethylbenzene conversion, thus lowering the quantity of recycle to the para-xylene recovery unit and concomitant processing costs.
Hydrogen generally is present in the conversion process reactants to aid in the reaction and maintain catalyst stability. Although xylenes may be isomerized in the absence of hydrogen under some circumstances with resulting cost savings, ethylbenzene conversion generally requires the presence of hydrogen. Two-stage processing units may be justified in some cases to obtain both high conversion and ethylbenzene yield. In any case, the search continues for more effective catalysts and processes.